FIG. 1(a) is a perspective view of a conventional rectangular asymmetric hybrid planar waveguide gas laser device 1 consistent with the prior art presented in U.S. Pat. No. 5,335,242 used to generate laser output wavelengths from 3 to 12 microns in length. To generate gain within the laser cavity formed between the resonator mirrors 2, 3, a Radio Frequency gas discharge is created between two metallic planar electrodes 4 often referred to as slabs; by exciting a Radio Frequency discharge in the narrow gap 5 with dimension A between the electrodes. For an overview of this technology refer to U.S. Pat. No. 4,719,639. The laser has vertical A and horizontal dimensions B different from each other, and laser resonator mirrors 2, 3 which are concave spherical mirrors to form a negative branch unstable resonator in the wider lateral free-space direction B, whereupon the beam is out-coupled past the hard edge 6 of the mirror 3 which has the lowest radius of curvature of the two resonator mirrors. In the much narrower transverse direction A the resonator mirrors 2, 3 in conjunction with the planar electrodes 4 form a waveguide. A plan view of the laser is given in FIG. 1(b) showing the free-space unstable direction, and a sectional view is shown in FIG. 1(c) showing the waveguide direction of the resonator. The following discussion outlines the limitations of conventional correction schemes to format the output beam from an asymmetric hybrid planar waveguide laser resonator, into a round and near diffraction limited beam; which is a prerequisite for most laser process applications.
Referring to FIG. 1(b), the internal resonator beam 7 is out-coupled from the resonator past the hard edge 6 of mirror 3, generating a beam 8 with a profile 9 in the free-space unstable direction which is often referred to as a top-hat. This near-field beam will accordingly need to be propagated into its far-field where-upon its profile 10 has a central maxima with adjacent side lobes which can then be removed using a spatial filter to give a Gaussian-type near diffraction limited beam profile. Propagation into the far-field is produced by either allowing the beam to naturally propagate, or to generate a focus using either a concave mirror or a positive lens. In general the choice of resonator in the free-space unstable direction is confocal in type, dictating that the out-coupled beam 8 in that direction is collimated. In order to achieve suitable output couplings from these types of lasers operating at a chosen wavelength in the range 3 to 12 microns, the beam width C in the free space direction is generally of a size such that the distances required to allow the collimated beam 8 to naturally propagate into the far-field, are of the order of many metres. This dictates that for most designs the out-coupled beam 8 in the free space direction must be focused, and therefore requires at least two additional external mirrors to turn the beam and fold it back along the length of the external structure of the gas laser discharge vessel. This method keeps the design as compact as possible, while also negating the need to use high focusing power optics to generate the free space far-field; whereupon large Irradiance levels would be present in the filtered side lobes and a very high degree of positional accuracy would be required for mounting of the spatial filter to avoid thermal damage.
Depicted in FIG. 1(c) is the waveguide direction of the resonator. As mentioned above, this direction is orthogonal to the free-space unstable direction of the resonator. The waveguide beam 11 is generated in the waveguide gap 5, with a width A from 1-2 mm, dictating that the exiting near-field waveguide beam profile 12 is highly divergent compared with the wider, collimated beam in the free space unstable direction, however, unlike the free-space unstable beam, the profile shape in both the near 12 and the far field 13 is near Gaussian, and therefore does not require spatial filtering. In general, the exiting waveguide beam 11 is allowed to propagate to a distance whereupon its size is equal to that of the free-space beam. The wave-front radius of curvature (divergence) of the waveguide beam is then corrected to match that of the free space beam using a cylinder mirror/lens or an angled spherical mirror, this correction occurring either prior to, or after the free-space beam has been spatially filtered. It is common practice to use the afore mentioned two folding mirrors that are used to fold the beam back along the length of the external structure of the gas laser discharge vessel, to perform some, or all of the correction in the waveguide direction. However, in order to achieve the required correction within the limitation of keeping the corrective scheme no longer than the length of the laser discharge vessel, it is often required that additional beam folds be used. Such designs often require additional frameworks and optical mounting systems, driving up costs and overall structural weight. Added to these issues, is the need to allow for variations in both the resonator beam characteristics and tolerances in the optical powers of the corrective system, dictating that the corrective system must be adjustable to some degree, both in the positioning and the angle of incidence of its component corrective optics.